There are two Warlock twins, Boris and Doris, who have made a deal with a celestial entity to cleanse the evil from the world. In one of the catacombs they are exploring they encounter a skeleton. Boris, with the higher initiative roll, reacts to the skeleton first and moves to the skeleton's left while preparing to fire an eldritch blast (using the Ready action) using the Repelling Blast invocation at the moment that his sister uses eldritch blast; this is intended as a way to apply a combo attack.
Doris remembers that skeletons are particularly weak to being crushed, so she gets an idea moving to the right side of the skeleton and firing an eldritch blast, also with repelling blast invocation, thinking that when she and her brother fire at the same time the skeleton will be crushed due to the opposed push by two Repelling Blast-empowered eldritch blasts.
For the sake of visualisation:
Which of these three possibilities happen?
- The skeleton moves to Boris's side due to the repelling blast and gets an attack of opportunity on Boris when he casts eldritch blast.
- The skeleton simply keeps its position.
- The skeleton takes damage as if it fell 20 feet due to the crushing force. (With 10 feet of a push from either side)